The invention relates generally to the field of programmable biomedical implantable devices and more particularly to external programmers for implanted cardiac pacers and neural stimulators.
A cardiac pacer is a life-sustaining therapeutic device which cooperates with the patient's heart by causing it to beat in a natural rhythm when the heart may othewise fail to provide adequate blood circulation. Commercially practicable cardiac pacers are powered by self-contained batteries and fully implanted in hermetically sealed or encapsulated containers in the patient's body, for example, in the shoulder region just beneath the skin. Electrical conductors called pacer leads run from the implant through an opening beneath the skin in a nearby vein and extend through the vein to the heart. The lead terminates in an electrode which contacts the interior wall of the heart muscle. Electrical stimulation pulses are generated by the cardiac pacer and applied via the pacer lead to the cardiac muscle to stimulate contraction, particularly when spontaneous natural heart action is absent. Along with the batteries, the implanted pacer includes electronic circuitry for generating the stimulation pulses. The pulses which are applied to the heart muscle have two fundamental variable characteristics or parameters: pulse intensity and pulse timing. Pulse intensity normally refers to the amplitude (pulse height) of the electrical current flowing to the heart muscle via the pacer lead during the pulse. Pulse timing refers to the duration (pulse width) of the pulse as well as to the time interval between pulses (interpulse period), which is related to the pulse rate, e.g., 70 beats per minute.
In early pacemakers the pulse parameters were fixed, established during manufacture. In 1972 the first digital programmable pacemaker, the Cordis Omnicor, was introduced to the market. The term "programmable" in this context means the ability to alter parameter values in the implant noninvasively from outside the body by transferring parameter data from an external programmer to the implant. See, for example, U.S. Pat. No. 3,805,796 to Terry et al. Digital pacing generally refers to the use of a digital counting chain driven by a high frequency oscillator to determine all the timing intervals. See, for example, U.S. Pat. No. 3,557,796 to Keller et al. The advantages of programmability once obtained were readily apparent. Since most cardiac patients are in a continually changing, and for older patients, usually declining state of health, the optimum pacer parameters must be changed frequently during the patient's lifetime. Without programmability such changes involved surgery or invasive techniques which ran significantly increased risk of infection due to the presence of the foreign body in the patient. This risk is eliminated by programmability. Secondly, a manufacturer does not have to proliferate an inventory of models of pacers which vary only in parameter values. Today these advantages are such that nearly all pacemakers are, and many argue all should be, programmable.
Nevertheless, programmability has not given rise to a universal pacer, i.e. a single model. Instead, with the continual march of technology and medical science, as well as the need for pacers which operate in limited pacing models such as ventricular inhibited pacing and AV synchronous or AV sequential pacing, a successful pacer manufacturer must contend with a growing host of pacers all implanted in living human patients and many programmable in different ways. The future definitely holds in store implanted pacers with telemetry for data communication with the outside world. New pacing modes, diagnostic techniques and temporary pacing treatments will also undoubtedly be discovered.
The challenge is to develop a programmer which is versatile enough to program all of these pacer products safely noninvasively and with a procedure as straightforward as the functional requirements.